744 BELL SYSTEM TECHNICAL JOURNAL 



when there are only a few lamps as, for instance, at highway inter- 

 sections. 



A Cardioid Directional MicroplwneJ R. N. Marshall and W. R. 

 Harry. A microphone is described which has uniform directivity 

 over a wide frequency range. This is made possible by placing in a 

 single instrument a dynamic type pressure microphone element and a 

 ribbon type "velocity" element, and electrically equalizing the out- 

 puts before combination. The resultant directional pattern is a 

 heart-shaped curve or cardioid, giving a fairly wide pick-up zone in 

 front and a substantial dead zone at the back of the instrument. 

 Because of the unusually rugged ribbon employed, the new microphone 

 is much less susceptible to wind noise than ordinary ribbon types. 

 Housed in an aluminum case, the microphone weighs only 3j lbs. 

 High output level, low impedance, and high quality, together with 

 the excellent directivity, promise to make the cardioid microphone an 

 important tool for the motion picture sound engineer. 



Fractional- Frequency Generators Utilizing Regenerative Modulation.^ 

 R. L. Miller. By the application of the principle of regeneration to 

 certain modulation systems, a generator of submultiple or other 

 fractional-frequency ratio may be obtained. 



A simple example is obtained by considering a second-order modu- 

 lator whose output is connected back to a conjugate input by means 

 of a feedback loop including an amplifier and a selective network. 

 If an input frequency /o is applied, it is found that a frequency com- 

 ponent fo/2 appearing in the feedback path will modulate with the 

 applied frequency to produce sidebands of fo/2 and 3/o/2. The net- 

 work and amplifier, being especially efficient for the frequency fo/2 

 and having a gain higher than the modulator loss, will reinforce this 

 component causing it to build up to some steady-state value. Similar 

 processes are possible by which greater submultiple ratios may be 

 obtained. 



Since the output wave is obtained by a modulation process involving 

 the input wave, it will appear only when an input is applied and then 

 bears a fixed frequency ratio with respect to it. Experiments show 

 that the ability of the generator to produce a fractional frequency is 

 independent of phase shift in the feedback path. Circuits are possible 

 in which the amplitude of the fractional-frequency wave will bear a 

 linear relation to the input wave over a reasonable range and at the 



^ Jour. S.M.P.E., September 1939. 

 8 Proc. I.R.E., July 1939. 



